Preheat the oven to 190C (370F). Line the muffin tins with paper cups.You can also use silicone muffin cups if preferred.
Prepare the chocolate mixture: In a large bowl combine dry ingredients, flour, sugar, cocoa powder and baking powder. In another bowl whisk the yogurt, oil, butter, egg and vanilla extract and add to flour mixture. Add the chopped chocolate. Spoon mixture into prepared muffin cups.
Mix well the cream cheese with sugar, egg and vanilla extract. Spoon mixture into the muffin cups on top of chocolate mixture. Gently swirl with knife to get the marbled effect.
Bake for 25-30 minutes until a toothpick inserted in the center comes out clean. Allow to cool over a wire rack before serving.